1. Field of the Invention
The present invention generally relates to reducing depleted organics in electroplating baths.
2. Description of the Related Art
Sub-quarter micron multilevel metallization is a key technology for the next generation of very large scale integration (VLSI) and ultra large scale integration (ULSI). The multilevel interconnects that lie at the heart of these integration technologies possess high aspect ratio features, including contacts, vias, lines, plugs, and other features. Therefore, reliable formation of these features is critical to the success of VLSI and ULSI, as well as to the continued effort to increase integrated circuit density, quality, and reliability on individual substrates. As such, there is a substantial amount of ongoing effort being directed to improving the formation of void-free sub-quarter micron features having high aspect ratios, i.e., features having a height to width ratio of about 4:1 or greater.
Elemental aluminum (Al) and aluminum alloys have conventionally been used as conductive materials to form lines, plugs, and other features in integrated circuit semiconductor processing techniques, as a result of aluminum's low resistivity, superior adhesion to silicon dioxide (SiO2) substrates, ease of patterning, desirable electromigration characteristics, and relatively high purity available at moderate costs. However, as circuit densities increase and the size of conductive features therein decreases, conductive materials having a lower resistivity than aluminum may be desirable. Therefore, copper and copper alloys are becoming choice metals for filling sub-quarter micron and smaller high aspect ratio interconnect features in integrated circuits, as copper and copper alloys have a lower resistivity than aluminum, and therefore, generate better resistance/capacitance time delay characteristics. Additionally, copper provides improved electromigration characteristics over aluminum.
However, a challenge with using copper in integrated circuit fabrication is that copper is not easily deposited into high aspect ratio features with conventional semiconductor processing techniques. For example, physical vapor deposition (PVD) techniques may be used to deposit copper, however, PVD copper deposition is known to encounter difficulty in obtaining adequate bottom fill in high aspect ratio features. Additionally, chemical vapor deposition (CVD) may be used to deposit copper, however, CVD suffers from low deposition rates, and therefore low throughput, in addition to using precursors that are difficult to manage. Additionally, copper is difficult to pattern with conventional semiconductor processing techniques, and therefore, copper must generally be deposited directly into features, where conventional aluminum techniques allowed for deposition and patterning of the conductive features. In view of these challenges, electroless and electroplating deposition techniques have become an attractive option for depositing metal, specifically copper and copper alloys, onto semiconductor substrates and into high aspect ratio features.
Conventional electroplating methods generally include positioning a substrate 101 on a substrate support member 102 in a face down configuration, i.e., the receiving surface 103 of the substrate support member secures the substrate 101 thereto such that the exposed surface of the substrate faces downward, as illustrated in FIG. 1. The substrate support member 102 is then lowered into a plating bath 104, which generally comprises an electrolytic solution. An electrical bias is then applied between the surface of the substrate and an anode positioned in the plating bath, which operates to urge metal ions in the plating solution, which may be copper ions, to deposit on the substrate surface. During non-processing time periods, i.e., when substrates are not being plated, the electrolytic solution is generally circulated through a continual path that includes a relatively small volume plating bath/cell 104 and a substantially larger volume storage cell 105. The storage cell 105, for example, may hold approximately 200 liters of plating solution, while the plating cell 104 may hold approximately 2 liters of plating solution. Additionally, the continual fluid path may include an electrolyte replenishment device 106 configured to replenish portions of the plating solution that may be depleted through plating operations.
Typical electrolyte solutions used for copper electroplating generally consist of copper sulfate solution, which provides the copper to be plated, having sulfuric acid and copper chloride added thereto. The sulfuric acid generally operates to modify the acidity/pH and conductivity of the solution, while the copper chloride provides negative chlorine ions needed for nucleation of suppressor molecules and facilitates proper anode corrosion. The electrolytic solutions also generally contain various organic molecules, which may be accelerators, suppressors, levelers, brighteners, etc. These organic molecules are generally added to the plating solution in order to facilitate void-free super-fill of features and planarized copper deposition. Accelerators, for example, may be sulfide-based molecules that locally accelerate electrical current at a given voltage where they absorb. Suppressors may be polymers of polyethylene glycol, mixtures of ethylene oxides and propylene oxides, or block copolymers of ethylene oxides and propylene oxides, for example, which tend to reduce electrical current at the sites where they absorb, and therefore, slow plating at those locations. Levelers, for example, may be nitrogen containing long chain polymers, which operate to facilitate planar plating.
During the plating process, copper ions are continually being removed and replenished to/from the electrolytic solution, and therefore, the copper concentration of the electrolyte inherently changes or varies over time. This concentration change may further be affected by volume depletion of the plating solution and/or dissolution of the anode. Additionally, plating operations also deplete the various organic molecules in the electrolyte solution, and therefore, the organic concentration also varies over time. For example, levelers are known to deplete/breakdown upon exposure to oxygen containing elements, i.e., ambient air, oxygen absorbed into the plating solution, oxygen molecules contained in the anode metal, or oxidation encountered during plating by incorporation into a growing film. This breakdown process generates free radicals in the plating solution, which are undesirable, as the free radicals can deposit on a substrate and contaminate the metal layer. Further, levelers are known to breakdown upon exposure to copper, copper alloys, and/or platinum, all of which are typical anode materials for electroplating systems. Similarly, accelerators and suppressors may also suffer from depletion/breakdown characteristics as a result of oxygen and/or metal exposure. Depletion of organics is not limited to processing time periods, as the electrolyte solution in electroplating systems is generally continually circulated through the plating cell, storage unit, and potentially a replenishment device during non-processing time periods. As a result of the circulation, the plating solution may be continually exposed to both oxygen-containing elements and the metal anode. Therefore, as a result of this exposure, the organic molecules in the plating solution are continually being depleted, even though the plating system is not in a plating or operational mode.
Inasmuch as the concentration of the organics in the plating solution and the concentration of the radicals generated by organic molecule breakdown process both have a substantial effect upon the efficiency and controllability of plating operations, replenishment of depleted organics in the plating solution, as well as maintaining specific organic concentrations is desired. Conventional plating systems generally provide a replenishment module configured to add fresh organics into the plating solution in order to replenish depleted organic molecules. However, conventional organic replenishment processes generally require time consuming organic molecule measurement processes, which decreases the accuracy of conventional organic replenishment processes, as the time duration required for measurements substantially decreases the accuracy of conventional organic replenishment processes and may cause an organic concentration variance. This variance in organic concentration may detrimentally affect the ability to accurately control conventional electroplating apparatuses.
Therefore, there exists a need for a method and/or apparatus for accurately replenishing organic molecules in an electroplating bath during plating operations. Additionally, there is a need for an apparatus and/or method for minimizing organic molecule depletion during non-processing time periods.